鄂尔多斯盆地南部三叠系延长组湖相重力流沉积细粒岩及其油气地质意义

Fine-grained,lacustrine gravity-flow deposits and their hydrocarbon significance in the Triassic Yanchang Formation in southern Ordos Basin

随着页岩油气勘探开发和相关领域研究的不断深入,细粒沉积物的搬运和沉积已成为当前沉积学研究的热点问题之一,但中国中生代湖泊环境中的泥质重力流沉积尚未引起应有的关注。通过岩心观察、薄片鉴定等手段及综合研究,分析了鄂尔多斯盆地晚三叠世湖相泥质重力流沉积特征,探讨了其形成机制与成因分类。鄂尔多斯盆地三叠系延长组湖相泥页岩结构类型多样,发育泥质块体流沉积、泥质碎屑流沉积、泥质浊流沉积和泥质异重流沉积等多种重力流沉积类型。按照泥质含量将重力流划分为砂质重力流、泥质重力流和混合重力流3种亚类,并根据成因将重力流划分为滑塌体、碎屑流、浊流及异重流等4种亚类;结合成因和泥质含量,将重力流沉积共划分为12种类型。滑塌岩、碎屑岩分布于三角洲前缘斜坡脚附近;浊积岩、异重岩广泛分布于三角洲斜坡至沉积中心。认为泥质沉积物可以在强水动力条件下搬运—沉积;重力流沉积细粒物质在湖相沉积中占据很大的比例;泥质重力流对泥页岩中的碎屑物质、黏土矿物及有机质的搬运和沉积起到重要作用,因而对于页岩油气的生烃、储集性能和压裂工艺研究具有重要意义。

With deepening research on the development of shale oil and gas development and related fields,transportation and deposition of fine-grained sediments have been a significant area of research in the stratigraphic community. However,muddy gravity-flow deposits in the Mesozoic lacustrine environment in China have received little attentions. Based on core examinations,thin section identifications and comprehensive analysis,current study investigate sedimentary features of muddy gravity-flow deposits in the Late Triassic southern Ordos Basin,and discuss their genesis and classifications. Various types of lacustrine mudstones or shales were recognized in the Yanchang Formation in the southern Ordos Basin,including muddy slumps,muddy debris flows,muddy turbidity currents and muddy hyperpycnal flows. Gravity-flow deposits are classified into three types,namely sandy gravity-flow deposits,muddy gravity-flow deposits and hybrid gravity-flow deposits. Based on mud contents and gravity flows＇ originations,four types of slumps,debris flows,turbidity currents and hyperpycnal flows were inferred. And then they are classified into 12 types based on origination and mud content. Slumps and debrites mainly occur near to toe slope of delta fronts,while turbidites and hyperpycnites are well developed in large extent from delta slopes to basin plains. It is considered that（ 1） mud can be transported and deposited by active hydrodynamics;（ 2） finegrained deposits created by gravity flows take a high percentage in lacustrine mudstones and shales;（ 3）muddy gravity flows are significant in transportations and depositions of clastic particles,clay minerals and organic matters,and are thus of great significance to hydrocarbon generating,reservoir capability and research on fracturing to unconventional oil and gas.